Laminated diaphragms are commonly used in pressure regulators and the like where pressure response and flexing are required. Such diaphragms normally consist of an inner layer of porous reinforcing fabric and outer layers of impermeable flexible sealing or elastomeric material. Different types of impermeable elastomers are used as well as different types of porous reinforcing material and the strength of the bond between the layers of the laminate varies with the materials of construction and the methods of manufacture. Total and complete bonding of the laminated layers one to the other is never possible and voids, however small, between the bonded surfaces are inevitable. Regardless of the selection of materials and bonding methods, and somewhat independent of the severity of use, the pressure of gases and saturated vapors against the diaphragms eventually permeates the elastomeric material and fills any voids between the layers of laminate that may be present. Experience has indicated that voids exist within the sheets of flexible elastomeric material used in these structures, either internally or along their surfaces. With some elastomers the voids can be eliminated by careful forming of the sheet material, but no degree of care can prevent the existence of voids or weakened places in the bonded interface of the laminates.
In a pressurized system as exists on both sides of such a diaphragm, the internal pressure of the laminate structure is the same as or tends to follow that of the external pressure as long as the system is stable or the changes are sufficiently slow to allow equilibrium to be established through permeation or breathing. When saturated vapors are involved in the pressurized system, changes in temperature also leave an effect on the pressure balance, which effect is less pronounced with gaseous medis.
During operation of the diaphragm, as the pressure increases suddenly, the voids tend to be compressed and no immediate damage to the diaphragm results. The internal pressure eventually comes to an equilibrium with the external pressure. This is generally true except where the bond interface between the laminates and around the voids is very weak, in which event sudden fluctuations in internal pressure of the voids causes the voids to become larger.
In those systems where the pressure remains substantially constant and the temperature fluctuates, any decreases in temperature are not necessarily damaging except to set the stage for increases in temperature which cause the gas or vapor in the voids to expand. The results can be disastrous causing rupture of the diaphragm when the internal pressure of the voids suddenly becomes greater than the external pressure of the system and insufficient time is allowed for the gas or vapors in the voids to escape.
A study by the instant applicants of the structural failures of a number of diaphragms which were constructed using an internal layer or porous reinforcing material covered on each side with a layer of a sealing elastomer, shows that the voids remain static when the internal and external pressures are equal, but, as the external pressure drops, the voids are enlarged and the elastomer bulges on the same side of the bond interface at the location of the void. Generally, a sympathetic bulge also occurs simultaneously opposite the bond interface in the elastomer on the other side of the diaphragm.
As flexing of the diaphragm continues, even though the bond interface between the elastomer and the reinforcing material is strong, the void may not enlarge but a large bulge or bubble can occur in the elastomer. Also, opposite the sympathetic bulge a second void or break in the bond interface develops. Two things happen, either the bond is broken and the void enlarged, or the bond holds and the elastomer is stretched to form a bubble. Usually both conditions ensue during operation of the diaphragm. The creation of a large bulge or bubble in the elastomeric layer of the diaphragm results in almost immediate failure of the device having the diaphragm as a component, because the bubble interferes mechanically with the operation or the diaphragm ruptures. Repeated enlargement of the voids and repeated stretching of the elastomer progressively weakens the entire diaphragm and failure results. As soon as differences in the bond strength at the interfaces exist in any part of the diaphragm, the remaining portions thereof are placed under more strain at each flexing or pressure change, leading to the creation of more voids and ultimate failure of the diaphragm. The presences of clamps, washers, support discs as forms of mechanical sealing of the diaphragm, or means for attaining mechanical movement therefrom, accentuates the problem regardless of the bond strength.
Furthermore, clamping of a diaphragm at its periphery, a common expedient in this art, creates a condition which promotes the formation of voids that eventually become co-extensive across the entire arc of the bond interface. It is of interest to note that the formation of a sympathetic bulge on the opposite side of a void is practically unavoidable. The size and incidence of such sympathetic voids appears to be a function of the permeability of the reinforcing material and the bond strength.